1. Field of the Invention
The present invention generally relates to an image processor, and more particularly, to an encoder and a decoder for HD Photo.
2. Description of the Background Art
An imaging device aiming at inhibition of color noise is disclosed in Japanese Patent Application Publication No. 2005-333251 (hereinafter referred to as patent literature 1). A photoreceiver of such an imaging device is provided with a three-color filter of Bayer array. Furthermore, such an imaging device has a detection unit and a correction unit which are realized as a function of a CPU. The detection unit compares a pixel value of a Gb pixel with an average pixel value of four Gr pixels positioned diagonally opposite to the Gb pixel, targeting raw image data stored in a buffer memory. When the average pixel value of the Gr pixels is greater than the pixel value of the Gb pixel, the correction unit replaces the pixel value of the Gb pixel with the average pixel value of the Gr pixels, thereby correcting color noise.
Microsoft Corporation recently proposes HD Photo as a still image file format that offers higher image quality than JPEG while requiring more simple circuit configuration and computation than JPEG 2000.
An encoder for HD Photo includes a frequency transform unit performing predetermined frequency transform (PCT), and a pre-filter performing predetermined prefiltering so as to reduce block artifacts. The frequency transform unit performs frequency transform with a pixel block having 4 pixels in column×4 pixels in row as a unit region for processing. The pre-filter performs prefiltering with a region which overlaps with plural unit regions for processing by the frequency transform unit as a unit region for processing, before frequency transform is performed. One can chose at will whether or not prefiltering is performed by the setting of an overlap coefficient.
A decoder for HD Photo includes a frequency inverse transform unit performing frequency inverse transform that corresponds to the above frequency transform, and a post-filter performing postfiltering that corresponds to the above prefiltering. The frequency inverse transform unit performs frequency inverse transform with a pixel block having 4 pixels in column×4 pixels in row as a unit region for processing. The post-filter performs postfiltering with a region which overlaps with plural unit regions for processing by the frequency inverse transform unit as a unit region for processing, after frequency inverse transform is performed. Similar to the above, one can chose at will whether or not postfiltering is performed by the setting of an overlap coefficient.
The details of HD Photo are disclosed in, for example, “HD Photo—Photographic Still Image File Format”, [online], 7 Nov. 2006, Microsoft Corporation, [searched in the Internet on 10 Oct. 2007], <URL: http://www.microsoft.com/whdc/xps/hdphotodpk.mspx>. The details of JPEG XR related to HD Photo are disclosed in, for example, “Coding of Still Pictures—JBIG JPEG”, [online], 19 Dec. 2007, ISO/IEC JTC 1/SC 29/WG1 N 4392, [searched in the Internet on 4 Mar. 2008], <URL: http://www.itscj.ipsj.or.jp/sc29/open/29view/29n9026t.doc>
In an imaging device disclosed in the above patent literature 1, the detection and correction units perform detection and correction of color noise, based on the pixel values of Gr and Gb pixels. However, luminance and chrominance signals coexist in the pixel values of the Gr and Gb pixels. Thus it is difficult to accurately remove only color noise without affecting luminance. Furthermore, the imaging device disclosed in the above patent literature 1 requires dedicated detection and correction units to be additionally implemented merely to inhibit color noise, which causes increase in cost.